1. Field of the Invention
The present invention relates to a system for locating a moving object. Although not exclusively, it is particularly appropriate to the location and guidance of missiles, for example anti-tank missiles.
2. Description of the Prior Art
From the U.S. Pat. No. 4,717,028, a system is already known for locating a moving object with respect to an axis, comprising an optical detector disposed in a fixed station and cooperating with an optical transmitter connected to said moving object, which is remarkable in that:
said optical detector comprises, on the one hand, a fixed matrix of photosensitive charge transfer elements (CCD matrix or similar), whose plane is at least substantially orthogonal to said axis and which is associated with an optical system adapted to form on said matrix an image of the environment of said axis in which said moving object is likely to move and, on the other hand, electronic means for controlling said photosensitive elements; PA1 said optical transmitter generates light flashes and synchronization means are provided so that said electronic control means deliver control pulses synchronized with said light flashes and initiate image taking. PA1 an optical transmitter connected to said moving object and generating light flashes, under the dependence of a first time base; and PA1 a photosensitive detector disposed in a fixed station and associated with an optical system observing the environment of said axis in which said moving object is likely to move, said photosensitive detector comprising a charge transfer and frame transfer matrix detector controlled by electronic means associated with a second time base, and said matrix detector comprising a photosensitive zone on which said optical system is able to form the substantially pinpoint image of said light flashes, the succession of the integration times of the images in said photosensitive zone and of the transfer times of said images therefrom being under the dependence of the second time base; PA1 to reduce the requirements concerning previous synchronization; PA1 to reduce the safety time margin taken on the integration, which improves the connection result; and PA1 to accept relative drifts of the time bases of the transmitter and of the detector.
Thus, with such a system, the position of the moving object with respect to the axis is given by the position of the photosensitive elements of the matrix, energized by the image of the light flashes, with respect to said axis.
It will be noted that the optical transmitter generating light flashes and connected to the moving object may be a light beacon carried thereby or else a simple mirror receiving light flashes from a fixed beacon and reflecting said flashes towards the optical detector.
In this known system, since operation is pulsed and not continuous, a particularly high signal/noise ratio may be obtained. In fact, it is possible to take pictures which have a duration equal to that of the flashes of the optical transmitter and which are synchronized in frequency and phase with the light transmission.
Thus, synchronization of the detector and the transmitter make it possible to operate said detector with a useful signal ratio close to the peak value of the power of the transmitter.
The transmitter-detector connection may then operate with a high signal/noise ratio, for a low mean power, which limits the consumption and the cost of the transmitter.
As is explained in this prior patent, synchronization between the detector and the transmitter may be obtained by permanent connection means or else by temporary connection means, said detector and transmitter then comprising means (time bases) for individually maintaining said synchronization.
In the first case (permanent connection means), synchronization may be provided by an immaterial connection or by a material connection. If it is an immaterial connection, of radioelectric type, it is expensive for it is necessary to integrate appropriate equipment in the moving object; in addition, such radioelectric connection is sensitive to jamming and to electromagnetic disturbances, which is a serious drawback, and may lead to loss of the moving object by the optical detector. If it is a material connection, it must have a high pass-band so as to allow a high synchronization frequency. Such a material connection can then only be formed by an optical fiber, which confers a high cost on the system. In any case, a material connection necessarily limits the radius of action of the moving object with respect to the optical transmitter.
Thus, it is often advantageous and even necessary for synchronization to be obtained using time bases synchronized by a temporary connection, previous to launching of the moving object, one of said time bases being associated with the detector and the other with the transmitter.
However, the errors in initial calculation of the period and the relative drifts of the two time bases introduce a temporary slipping of the integration time of the CCD matrix with respect to the flashes of the transmitter. It is then necessary to extend the integration times of the CCD matrix so that each flash is always contained in the corresponding integration time interval, during the flight time of the missile. But then, the signal/noise ratio is not as good and the result obtained may be scarcely better than in the case of a continuous transmitter.